The present invention relates to a film treatment device and a wrapping apparatus using a heating resistor to cut a wrapping film.
In prior art, various types of wrapping apparatus for wrapping a web of plastic film around an object to be packed are known. One of these types of wrapping apparatus comprises a film dispenser arranged to circulate around the object to be wrapped along a ring-like track while the object to be wrapped remains immovable during the wrapping operation. The movement of the film dispenser may be controlled by a circular guide rail or the film dispenser may be attached to a rotatable arm. The film runs from a roll in the film dispenser and is wrapped around the object. There are also wrapping apparatus in which the film dispenser is fixedly mounted on a column frame on a base and the object to be wrapped is circulated in relation to it on a rotating floor.
In conjunction with this type of wrapping apparatus, various film treatment devices are known e.g. from specifications WO 99/19216, U.S. Pat. No. 4,993,209, FI 91624 and EP 0493 940. Typically, a film treatment device comprises gripping means which can hold the film web at the beginning and end of the wrapping operation. Moreover, the film treatment device comprises a cutting device for cutting the film web at the end of the wrapping operation, and hot sealing means for seaming superincumbent film layers together near the cut-off line so that the end of the web remains fastened to the film layer below it.
The cutting and seaming means presented in specification WO 99/19216 comprises an infrared heat radiator disposed in a casing having apertures through which infrared radiation is directed through the film at a heat absorbing cutting and seaming stop block, which is thus heated and cuts the film, seaming the film layers placed against each other together. The use of an infrared heat source has the advantage of requiring practically no maintenance. However, the use of an infrared lamp involves the drawback of a relatively high price, requiring a high electric power and a bulky structure. Moreover, both the seaming and the cutting of the film are performed using the same heat source. The timing of the seaming and cutting operations cannot be adjusted independently of each other. If a wrapping film plastic type requires different cutting and seaming temperatures, then these cannot be adjusted independently of each other in each case.
In the wrapping apparatus presented in specification U.S. Pat. No. 4,993,209, a seaming and cutting device is used in which the film is cut and seamed by blowing hot air at the film. The problem is a varying seaming quality and an inaccurate cut-off line. Heating the air to a sufficient temperature and bringing it to the cut-off line is also difficult and disadvantageous in respect of energy economy.
Specifications FI 91624 and EP 0 493 940 present devices for cutting a film in which the film is cut by a cutting blade. The problem with a cutting blade is its risky nature. When the operator is changing the film roll and feeding in a new film to allow the gripping means to take hold of it, his hand may touch the cutting blade placed close to the roll.
Anyway, the closest example of prior-art technology is a film treatment device known from the applicant""s xe2x80x9cOctopusxe2x80x9d wrapping machine in which the cutting device comprises an electric resistance wire held fast at both ends by fastening elements and extending freely without support between the fastening elements so that it is surrounded by an air space on all sides. At the end of the wrapping operation, the film web is pressed against the resistance wire and an electric current is passed through the wire, the wire being thus heated so that it cuts the film web.
The problem is that a repeated fatigue inducing load is transmitted from the film web to the areas near the points of attachment of the resistance wire, producing with time a fatigue fracture and breaking the wire. In addition, the thin resistance wire (diameter about 0.6 mm) is susceptible to breakage if the operator of the machine incautiously presses it when fitting the end of the film web to the gripping means placed near it. A further problem is that, as the film web is bent over the resistance wire, the area of contact between the film and the circumference of the resistance wire is fairly large, which means that the entire contact area has to be burned in order to cut off the web, thus producing noxious impurities in the air. If e.g. a film web of a width of 0.5 m is bent into an angle of 120 degrees over a 0.6 mm resistance wire, then for each cut an area of 3.15 cm2 of plastic needs to be burned, producing smoke. A further problem is that plastic crust adheres to the wire, deteriorating its functional quality.
The object of the invention is to eliminate the above-mentioned problems.
A specific object of the invention is to provide a film treatment device in which the durability of the electric resistance wire has been improved so that it practically requires no maintenance at all. A further object of the invention is to provide a film treatment device that is safer and more reliable than earlier devices. An additional object of the invention is to provide a film treatment device in which the amount of plastic burned in the cutting operation is minimized.
According to the invention, the film treatment device comprises a supporting element arranged to support the electric resistor laterally substantially over the entire length of the electric resistor between the fastening elements. The supporting element constitutes a back stop for the electric resistor against the pressure of the film, keeping the resistor continuously firmly immovable in position.
The invention has the advantage that the arrangement for supporting the electric resistor makes it possible to achieve the same advantages as in the case of a device using an infrared radiator but in a simpler manner and at a lower cost. The susceptibility to damage of the electric resistor is reduced and its useful life considerably prolonged while a safer and more reliable device is achieved. In addition, the invention makes it possible to use an electric resistor in which the part coming into contact with the film is as narrow as possible so that the amount of plastic material burned in conjunction with the cutting is minimized. Besides, no plastic material will adhere around the electric resistor.
In an embodiment of the device, the electric resistor is of a band-like structure and is in cross-section substantially the shape of a flat rectangle. The supporting element comprises a slot designed to receive the band-like electric resistor partially inside it. The walls of the slot support the broad sides of the band laterally from both sides while the bottom of the slot supports the narrow edge of the band.
In an embodiment of the device, the slot has been fitted to receive the electric resistor inside it with a clearance to allow repeated thermal elongation and contraction of the electric resistor band.
In an embodiment of the device, that part of the supporting element which supports the electric resistor against the pressure of the film web is of a curved shape to give the electric resistor a curved shape, and the ends of the electric resistor are fastened to the supporting element so that at least one end is connected to the supporting element via a spring. The curvature of the electric resistor and the tension created in it by the spring are necessary to keep the electric resistor in the slot. Due to the curvature, the electric resistor is strained into a curved shape, which means that a force component directed at the support, such as the bottom of the slot, is generated in it, pressing the electric resistor toward the bottom of the slot and preventing it from slipping out of the slot when subjected to a lateral force that tends to pull it out of the slot. Without the curvature, the electric resistor might easily get out of the slot.
In an embodiment of the device, the electric resistor band comprises an outer edge which cuts the film, and an inner edge remaining inside the slot and resting on the bottom of the slot, which has a curvature designed to bend the electric resistor band into a curved shape.
In an embodiment of the device, the supporting element and the electric resistor are so fitted in the device that, while the electric resistor bends the film web stretched over it, the supporting element also bends the film web at points located on opposite sides of the electric resistor at a distance from its outer edge, so that after being cut, the film web tends to bend clear of the electric resistor.
In an embodiment of the device, the supporting element is made of an electrically insulating, substantially rigid and heat resistant material, e.g. phenol formaldehyde.
In an embodiment of the device, the thickness of the electric resistor band is of the order of about 0.2 mm.
In an embodiment of the device, the film treatment device comprises gripping means for gripping the film web.
In an embodiment of the device, the film treatment device comprises seaming means for seaming superimposed film layers together by means of heat.